Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

The description explicitly discloses the key behavioral side effect: 'setting a new primary unsets the previous one', which goes beyond the annotations (destructiveHint, idempotentHint). It also states the 'already-registered' precondition and the one-per-user constraint, providing concrete details about how the tool changes state.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

The first three sentences are concise and front-loaded, but the description then includes a large block of boilerplate about 'Powered by CorpusIQ' and a data accuracy contract. This generic text is unrelated to the tool's specific purpose and significantly inflates the length, making the description less concise and focused.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a simple one-parameter tool with no output schema, the description covers the core concepts: what primary means, how it affects other tools, the constraint of only one primary, and the unsetting behavior. The annotations handle mutation/destructiveness, so the description is complete enough for an agent to select and invoke correctly.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The input schema has 100% coverage with a clear description for channel_id ('Channel ID (UC...) of an already-registered brand to mark as primary'). The tool description repeats 'already-registered' but adds no new meaning beyond the schema, so the baseline score of 3 is appropriate.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description uses a specific verb ('Change') and resource ('which already-registered Brand Account is the user's primary'), clearly distinguishing this tool from siblings like add_my_youtube_brand_channel and remove_my_youtube_brand_channel. It also explains the concept of 'primary' and its role as the default target for per-channel tools, reinforcing its unique purpose.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

The description provides clear usage context by stating that the primary brand is used when no channel_id is supplied, and that setting a new primary unsets the previous one. It implies a prerequisite of an already-registered channel, but it does not explicitly name alternatives or state when not to use the tool.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.